It maybe algae bloom. :T It can be caused by a few things: suspended microscopic algae floating around, excess light especially direct sunlight, too many fish which means high ammonia, or excess nutrients.

If it is algae bloom I really suggest you cover your tank with black paper, so no light can go through for three days or more, this will kill the algae. Water changes can help but it won't eliminate algae bloom.

Definitely looks better :) For 2.5 gall i make 1-50% and 1-100% water changes a week. You don't have to have filter for 2.5 gall. And if you will have filter make sure it not too strong. One of my betta stock in the filter.

The filter arrived, but even on the lowest setting it is too strong for my fish. I baffled it with some stalkings and this seems to work okay to keep the current out but I am worried it will burn out the filter.

Guppy seems to like it though, he is resting on top of it. first night he is not in his hammock.

I hope the heater will be less problematic.

Should I keep the light off for a few days altogether to kill the algae. I am feeling a lot of slime on the walls and surfaces even though I just washed them this morning, is this because of the algae or is there another problem?

Will the filter help with the algae problem?

I was under the impression that a filter is essential to extending fish life, but now I hear you don't even need one. I am so confused, I am not sure I am doing anything right. I don't want the little guy to be unhappy or stressed. :(

Filters are optional. I personally don't like them because i had accident,but other people have them. For 2.5 gall i prefer to do water changes like i wrote and i really don't need filters.

Is your fish show any sign of stress?

Your algae probably from the light. Not sure since my tanks doesn't have light. But i know in the doctor office that i am working he bought light bulbs for his tank. And those new light bulbs are stronger than he had before and his algae grow so fast. Is your tank on straight light?

Also even if you have light in your tank you don't need to keep it 24/7. I think it will stress fish. Specially at night turn it off.

I hope someone else will help you since i don't have filters, i don't cycle tanks i don't know anything about that. But if your tank cloudy again i would change water again. Do another 50% water change like you did before.

Hi Arena, welcome to the site!
1. Filter isn't mandatory for a 2.5g tank, but it will help a bit with cleaning water in between water changes...as stated above, 1 50% and 1 100% water change per week is highly recommended.
2. Your betta will get used to filter. Any changes made will probably upset or stress your betta, but give him some time to acclimate, and he will be fine.
3. Baffling your filter will not burn it out if done appropriately. Look at baffling threads on housing, decorations part of this site. You can also find good directions online...google 'baffling aquarium filters'...might even find directions for your specific filter. :)

4. To prevent algae bloom is a bit more difficult...cleaning on regular basis with wiping down sides of tank and vacuuming gravel is a very good start. Does your tank get sunlight or have high output lights? Too much light can start algae bloom. Also, too many nutrients can cause algae bloom, so too much ammonia, nitrate, nitrite can cause an algae bloom. Good start w/the cabomba (i think...or hornwort)....if you have plants to compete w/ algae, you can sometimes prevent a bloom. Fast growing stem plants like cabomba, hornwort, creeping Charlie, cardamine and floating plants like dwarf water lettuce, duckweed, and Amazon frogbit will all soak up a lot of nutrients from water, thus cutting off nutrient supply to algae! Unfortunately, many stem plants need a lot of light and regular lighting from aquarium hood doesn't always cut it.

**you are not doing anything wrong! In fact, you are doing a great job!